1. Field of the Invention
This invention relates to an ophthalmological diagnosis apparatus, particularly to an ophthalmological diagnosis apparatus in which a laser beam of prescribed diameter is projected at a region of the eye fundus to produce a laser speckle pattern formed by light scattered from tissues of the eye fundus, and movement of the laser speckle pattern is photoelectrically detected through a minute circular detecting aperture as fluctuation in the light intensity of speckles at an observation plane, thereby producing a speckle signal whose photon correlation function is calculated to determine the blood flow state in the eye fundus tissue.
2. Description of the Prior Art
Conventional laser Doppler methods of measuring blood flow in retinal and other tissue by illuminating the eye fundus with a laser beam include those described in "Investigative Ophthalmology," vol. 11 No. 11, p 936 (November 1972) and "Science," vol. 186 (November 1974) p 830, and in Japanese Unexamined Patent Publication Nos. 55-75668, 55-75669, 55-75670, 52-142885 (corresponding to GB 13132/76 and U.S. Pat. No. 4,166,695), 56-125033 (corresponding to GB 79/37799), 58-118730 (corresponding to U.S. Pat. No. 4,402,601) and U.S. Pat. No. 4,142,796. However, these laser Doppler methods involve the use of a high precision optical system, are complicated to use and provide results which lack repeatability and reliability, which hinder the practical utilization of the method.
In order to overcome the aforementioned drawbacks the present inventors have adapted laser speckle methods used for blood flow measurement in skin and the like (such as the methods described in Japanese Unexamined Patent Publication Nos. 60-199430, 60-203235 and 60-203236 and in "Optics Letters," vol. 10 No. 3 (March 1985) p 104) for ophthalmological applications involving evaluating the state of the blood flow in tissues of the eye fundus, and have filed the following related patent applications: Japanese Unexamined Patent Publication Nos. 62-275431 (U.S. Pat. No. 4,743,107 and EPC 234869), 63-238843 (EPC 284248) and 63-242220 (EPC 285314).
In the methods described in these publications with respect to eye fundus measurements, a detecting aperture is used to extract time-base fluctuations in the intensity of speckles formed at an optical Fourier Transform plane with respect to the eye fundus, or at the Fraunhofer refraction plane, or at an image plane (or a magnified image plane) that is conjugate with respect to the eye fundus.
If the large diameter of the detecting aperture is too large in comparison with the averaged diameter of the speckles at the detection plane, the averaging of the speckles ( also called integration effect ) by which the intensity fluctuation thereof at the detecting aperture is canceled. This causes the detected speckle signal to have a reduced contrast and a degraded S/N ratio.
If the diameter of the detecting aperture is smaller than the averaged diameter of the speckles, on the other hand, the signal has a very low intensity, thus making measurement itself difficult. The criterion is, therefore, such that the aperture diameter should be selected so as not to be too large or small in comparison with the averaged speckle diameter or the aperture diameter should be set so as to be substantially equal thereto. Because no decision is made based on definite criterion, the conditions relative to the detected light quantity or signal contrast are not constant and an optimum condition is not obtained.
On the other hand, a point-like detecting aperture is recommended for speckle velocimetry so as to enable precise reproduction of the speckle intensity fluctuation, as is disclosed in "The Review of Laser Engineering" (The Laser Society of Japan) Vol. 8, No.2 (1980, March) p.37 and No. 3(1980, May) p.3 or "Applied Physics" (Springer-erlag) Vol. 25(1981) p. 179. In fact, a definite aperture must be used to obtain a certain amount of light. No suggestion is, however, made as to how an optimum detecting aperture should be set.
Since the intensity of the speckle pattern obtained by light scattered from the eye fundus is very low, the photon correlation method is required for measurement. This method is, however, not effective when an optimum condition is not set to take into account the light quantity and signal contrast.
With respect to the shape of the detection aperture, proposal was made of a rectangular aperture, which, however, didn't provide advantages because, instead of translational motion, boiling motion of random blinking speckles dominates at the detection plane even if blood cells flow in a predetermined direction in the blood vessel. It has thus been found that a circular detection aperture is preferable for random motion.
On the other hand, the photon correlation function is impractical when the measured data have no sufficient light quantity and contrast because no convergence of correlation data is expected. The measured data of poor convergence were evaluated by visual observation, which was, however, ambiguous and subjective. Thus, there was a problem that the same data were employed or not employed because they visually converge or don't.